The invention relates to a method of testing the insulation of a high-voltage d-c cable and, more particularly, to a method of testing the ability of such insulation to withstand a voltage higher than the normal operating voltage of said cable while the cable is carrying high continuous currents.
The usual high-voltage d-c cable comprises a conductor that is normally operated at a predetermined voltage with respect to ground, a metal sheath surrounding the conductor that is normally at ground potential, and dielectric material between the conductor and sheath that is stressed by the voltage present between the conductor and sheath. It is sometimes necessary that this insulation be tested at a voltage considerably higher than the normal operating voltage. To make such test more meaningful, it is sometimes also necessary that this higher voltage be applied to the insulation for an extended period and, particularly, for an extended period while the cable is carrying high continuous currents.
A possible energization source for providing the desired current and voltage for such testing is the source of the system in which the cable is normally connected. While such a source can provide the desired current, it usually cannot supply the full higher voltage that is desired. It is possible to add a supplemental source in series with the main source to provide a higher total voltage, but this approach has certain disadvantages. First of all, the supplemental source can be relatively expensive; and, secondly, the entire system, rather than only the cable, is exposed to the higher total voltage of the modified source, and this could lead to a damaging failure in some part other than the cable.